1. Field of the Invention
The present invention relates to a molding die, and more particularly to a molding die for molding a synthetic-resin product having an inward curved or bent edge, such as an automotive engine hood or an automotive bumper for automobiles.
2. Description of the Related Art
Various molding processes have been used, including injection molding, compression molding, flow molding, etc., for molding synthetic resin material products. Dies for use in such molding processes need to be highly durable and capable of molding complex shapes in order to form mass-produced moldings.
When a resin molding having an inward bent edge, such as an automobile bumper, is to be molded by dies, since the synthetic resin material flows in a reverse direction at the bent edge, a slide core for molding the bent edge has to be used, in addition to a pair of dies, i.e., a fixed die and a movable die.
If the mating surfaces of the fixed die, the movable die, and the slide core are not sufficiently accurate, then the synthetic resin material tends to develop burrs at mating surfaces on the resin-molded product, while also leaving parting lines, which are caused by mating surfaces of the fixed die, the movable die and the slide core, on the outer surface of the resin-molded product. Thus, the outer surface of the resin-molded product becomes lower in quality.
Various technical approaches have been proposed to ensure that the mating surfaces of the fixed die, the movable die, and the slide core are accurate enough.
For example, Japanese Patent Publication No. 58-033091 discloses a method of and an apparatus for manufacturing a synthetic resin product having an inward bent edge. According to the disclosed method and apparatus, pressing cylinders are provided along directions in which a slide core is movable, and also along directions perpendicular to the direction in which the slide core is movable. Consequently, the slide core can be pressed strongly toward the fixed die and the movable die, in order to ensure that the mating surfaces of the fixed die, the movable die, and the slide core are accurate enough.
A molding die for manufacturing a synthetic resin product, which is disclosed in Japanese Patent Publication No. 58-033091, employs a cylinder as a pressing means for ensuring that the mating surfaces of the fixed die, the movable die, and the slide core are accurate enough. Since the pressure applied to the slide core depends on the capability of the cylinder, it may be necessary to use a cylinder having a high pressing capability, or even a plurality of cylinders, depending on the pressure under which the molten synthetic resin is injected into the cavity. However, according to the disclosure of Japanese Patent Publication No. 58-033091, the molding die itself has to be sufficiently large in size, because the cylinder or cylinders must be disposed within the molding die. Furthermore, inasmuch as the movable die is pressed by the cylinder, the cylinder imposes limitations on the position where the movable die is installed. The molding die also is likely to lose desired mechanical strength at the position where the cylinder is installed, or the molding die may fail to produce necessary die fastening forces.